The invention relates to batteries and battery operated devices. More particularly, the invention relates to a method and apparatus for determining the condition of a battery. The invention also relates to a method and apparatus for detecting a low battery condition in a device that uses a battery.
For battery powered devices, it is often useful to know the condition of the battery. More particularly, it is important to know when the battery is almost drained. This can be important in avoiding data loss or in avoiding loss of functionality while the device is being used. For example, failure to indicate a low battery condition of a battery operated computer could result in data loss if the remaining power is insufficient to save the information and exit the application. Battery operated devices are sometimes used in locations where replacement batteries are not readily available. Also, battery operated devices are sometimes used in applications where time required to change batteries can be too long. This is true, for example, with battery operated cameras, with medical devices, or other devices having time sensitive uses. Knowledge of the remaining battery capacity of a battery operated medical device could be crucial in a medical emergency.
Various battery condition detectors are known in the art. One type of battery condition detector keeps track of transfer of charge. Another type of battery condition detector keeps track of use time of the device. Keeping track of use time or charge transfer results in an inaccurate determination of battery condition. Batteries lose charge even when not in use. In addition, the battery""s temperature can affect its capacity and useful life. Furthermore, monitoring the electrical characteristics of a battery, such as voltage or resistance, often does not provide enough information regarding the capacity of the battery.
Electrotherapy devices are used to provide an electric shock to treat patients for a variety of heart arrhythmias. For example, external defibrillators provide relatively high-level electric shocks to a patient, usually through electrodes attached to the patient""s torso, to convert ventricular fibrillation (xe2x80x9cVFxe2x80x9d) or shockable ventricular tachycardia (xe2x80x9cshockable VTxe2x80x9d) to a normal sinus rhythm. Similarly, external cardioverters can be used to provide shocks to convert atrial fibrillation to a more normal heart rhythm. Many electrotherapy devices are powered by batteries. Reliably indicating a low battery condition well before depletion is critically important in electrotherapy devices.
Prior art electrotherapy devices provide an indication of a low battery condition and a depleted battery condition. Stopping an electrotherapy treatment to replace a battery can have a detrimental effect on the patient being treated. Many prior art electrotherapy devices fail to reliably indicate battery depletion. The actual amount of battery capacity remaining when prior art devices indicate a low battery condition can vary with the ambient temperature, battery manufacturing variances, battery discharge history, battery recharge history, etc.
Battery capacity detectors and indicators are known in the art. Most prior art battery capacity indicators are based on one or both of the following two methods: (1) measurement of a battery parameter, and (2) cumulative measurement of battery output. The measurements are typically made in absolute, not relative, terms. The prior art battery capacity indicators also sometimes include some way to compensate for the environmental effects of time and temperature.
One approach is shown in U.S. Pat. No. 5,250,905 to Kuo et al. The Kuo et al. patent discloses a battery capacity indicator for a primary battery, such as a standard alkaline xe2x80x9cAAxe2x80x9d battery, in which a much smaller indicator battery is connected in parallel to the main battery. The indicator battery is designed to have a much lower capacity and a correspondingly much higher impedance than the main battery. The anode of the indicator battery disappears during use, revealing a message such as xe2x80x9cReplacexe2x80x9d when the indicator battery and the main battery have discharged to the point where the main battery is nearly depleted. Because the indicator battery is very different from the main battery in design and composition, it is difficult to match the indicator battery and the main battery so the main battery is nearly depleted when the xe2x80x9cReplacexe2x80x9d message appears.
U.S. Pat. No. 5,596,278 to Lin discloses a condition indicator assembly including an electrochemical indicator battery connected in series to an auxiliary battery. The condition indicator assembly is connected in parallel to a main battery being tested.
U.S. Pat. No. 5,483,165 to Cameron et al. discloses a battery system and method in which a sense battery is substantially identical to, and connected in series with, the main battery. A predetermined additional load is placed on the sense battery to discharge it at a higher rate than the main battery. The voltage of the sense battery is monitored to determine when it is nearly depleted. A predetermined amount of capacity will remain on the main battery at this point. One drawback of the Cameron et al. approach, however, is the extra circuitry needed to accelerate the discharge of the sense battery.
Other battery capacity indicators are shown in U.S. Pat. No. 4,259,639 to Remirie; U.S. Pat. No. 4,725,784 to Peled et al.; U.S. Pat. No. 5,162,741 to Bates; U.S. Pat. No. 5,065,084 to Oogita; and U.S. Pat. No. 5,130,659 to Sloan.
The specifications of the patents described above, as well as those referenced below, are hereby incorporated herein in their entirety by reference.
The invention provides a battery system having a sense battery cell with a smaller capacity than a main battery cell.
One aspect of the invention provides a battery system including a main battery cell, a sense battery cell connected in series with the main battery cell, and a battery capacity indicator monitoring a parameter of the sense battery cell. The sense battery cell has a capacity less than the main battery cell""s capacity.
Another aspect of the invention provides a method of determining a battery condition in a battery operated device. The method comprises providing a battery system. The battery system includes a main battery cell and a sense battery cell. The sense battery cell has a capacity less than the main battery cell""s capacity. The main battery cell and the sense battery cell are connected in series. A parameter of the sense battery cell is monitored. A condition of the main battery cell is determined from the monitored sense battery cell parameter.